This invention relates to a mobile station or a mobile communication terminal and, in particularly, to a mobile station for use in a code division multiple access (CDMA) communication system which simultaneously carries out a plurality of communications using signals having the same frequency band by means of spread spectrum technique.
As a cellular mobile radio communication system, a wide variety of multiple access systems have been heretofore proposed and be adopted in the world. Among others, a recent tendency has been directed to a cellular mobile radio code division multiple access (CDMA) communication system which has a specific spreading code assigned to each channel and which will be simply called a CDMA communication system. In such a CDMA communication system, a modulated wave of an identical carrier frequency which is spread by each specific spreading code is transmitted as a radio signal from a transmitter side to a receiver side. Responsive to the radio signal, a CDMA receiver in the receiver side carries out synchronization operation by the use of each specific spreading code to identify a desired channel. In order to distinguish the channels from one another, different spreading codes are used to identify radio channels between the base station and the mobile stations.
In addition, the radio signal is received through a plurality of paths, namely, a multipath in the CDMA communication system and, therefore, multipath fading should be removed from the radio signal in the CDMA communication system by accurately detecting a predetermined signal such as a synchronization signal and/or a pilot signal.
Furthermore, it is to be considered in the cellular mobile radio communication system that each mobile station is moved in radio service areas or cells from one to another with communication kept between each mobile station and base stations. In this event, the base stations must be switched from one to another without interrupting communication with the mobile station.
Mobile communication terminals have become into use because of convenience. The mobile communication terminals generally introduce the above-mentioned CDMA communication system.
Various mobile communication terminals are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. 2001-54,162 or JP-A 2001-54162 discloses “TERMINAL FOR MOBILE COMMUNICATION” to minimize a delay buffer to adjust deviation in reception timing of a signal sent from a plurality of base stations in the case of conducting hand-over through site diversity reception with respect to a terminal for mobile communication. According to JP-A 2001-54162, a TDHO calculation section measures a time difference between a transmission timing of a communication channel with a hand-over source base station and a reception timing of a perch channel of a hand-over destination. A Tlast decision section decides a lowest reception timing on the basis of an adjustment value of a delay buffer that adjusts a deviation from reception timings of communication channels from each base station in a site diversity state. A CPU reports a value closer to a slowest retiming Tlast among received signals from each base station in the above time difference TDHO and a value to/from which a round unit by one symbol is added/subtracted as time difference information of the reception timing of the peach channel.
Japanese Granted Patent Publication of Tokkyo No. 2,947,279 or JP-B 2947279, which corresponds to EP0977378, discloses “Spread spectrum communication system and handover method therein.” According to JP-B 2947279, a spread spectrum communication system comprises at least one mobile station and a plurality of base stations for communicating with mobile stations in sites thereof by a spread spectrum scheme. Each of the base stations includes accumulation means for accumulating a reception timing difference indicating a difference in reception timing between a transmission signal a mobile station in a base station in an adjacent site and a transmission signal from the mobile station in a self-station, and calculation means for obtaining a reception timing of the transmission signal from the mobile station in the self-station by using reception timing difference between the self-station and a handover source base station in an adjacent site, which is accumulated in the accumulation means, when the self-station become a handover destination base station upon handover of the mobile station between adjacent sites.
Japanese Unexamined Patent Publication of Tokkai No. 2002-152,791 or JP-A 2002-152791 discloses “PHS(R) HANDING-OVER METHOD AND PHS(R) TERMINAL DEVICE” to shorten call interruption time attendant on handing-over. According to JP-A 2002-152791, when a PHS(R) terminal is to switch a radio zone, by selecting the radio base station of best conditions out of access timing synchronously secured by much more radio base stations, an environment in which a large number of other radio base stations securing synchronism exist is selected. Further, when the quality of communication with one radio base station is deteriorated in such a satisfactory environment, a communication line with the next radio base station is set while securing a communication line with this radio base station.
U.S. Pat. No. 5,345,448 issued to Keskitalo, Ilkka, which corresponds to EP0568212, discloses “Procedure for the handover a radio connection.” According to Keskitalo, in a digital Time Division Multiple Access radio communications network that includes a number of Base Transceiver Stations, and several Mobile Stations, a handover can be accomplished so that the parameters required in identifying a Mobile Station, and the frequency data, the data of the time interval to be used, as well as the frame number data of the channel are given to a new Base Transceiver Station to which a Mobile Station will be moved. The new Base Transceiver Station is tuned to listen to the channel determined by the data it received. The former Base Station transmits a handover command to the Mobile Station containing the frequency data, the data of the time interval to be used, as well as the frame number data of the channel to which the new Base Station has moved. After the command the Mobile Station turns the trafficing directly to said channel. The timing measurements between the Base Station and the Mobile Station are not required and any tuning correction is carried out by the channel equalizer.
Japanese Unexamined Patent Publication of Tokkai No. 2001-211,471 or JP-A 2001-211471 discloses “PACKET EXCHANGE REMOTE MOBILE RADIO COMMUNICATION SYSTEM USING MORE EFFICIENT HARD HANDOVER” to improve communication quality by providing a hard succession configuration in which the interruption of connection to a mobile object is small/short regarding a general mobile telephone system (UMTS) operating on the basis of a packet exchange theory, an overall system (GSM) for mobile communication or the other remote mobile radio communication system. According to JP-A 2001-211471, in this packet exchange remote mobile radio communication system such as the UMTS or the GSM, a target RNC requests a direct link to an operating controller for in hard handover about a mobile object, the controller transmits an uplink frame from the mobile object to both a TRNC and a core network, the network transmits a downlink frame to the TRNC. The TRNC decides the time when the hanover is completed and reconstructs a frame when necessary.
In a conventional CDMA type mobile station, it may be impossible to nonnally carry out communication using a dedicated physical channel because chip offsets have different values between a radio base station and the mobile station in the manner which will later be described in conjunction with FIGS. 4A through 4C.